The present invention relates to multi-stage compressors and, more particularly, to liquid injection control for reducing temperature increase in multi-stage turbocompressors.
As is well known, when work is done on a compressible fluid such as, for example, steam, the temperature of the compressible fluid increases. Four problems can result when the temperature increase is excessive:
1. The temperature difference between inlet and outlet may exceed the maximum temperature difference which can be handled in a single compressor body; PA0 2. Commonly used materials must be replaced with exotic (expensive) materials to withstand the temperatures near the outlet; PA0 3. The work required to compress the steam is unnecessarily increased; and PA0 4. The steam delivered from the outlet may be excessively superheated (temperature above its saturation temperature) for satisfactory use in subsequent processes.
A six-stage turbocompresssor, for example, receiving steam at a temperature of, for example, about 180 degrees F. may increase the steam temperature to about 750 degrees F. in the process of compressing it to about 75 PSIA if no steps are taken to cool the steam in the process of compression. From a practical engineering standpoint, a temperature difference of this magnitude between inlet and outlet exceeds the temperature difference which can be sustained by a compressor in a single housing. One solution, of course, is splitting the compressor into two parts in separate housings. This solution, besides almost doubling the cost of such an apparatus, fails to solve the problems described in succeeding paragraphs.
Excessive temperatures in final compressor stages may obviate the use of common materials for gaskets and metals. For example, at a temperature of 750 degrees F., iron or carbon steel pump bodies and impellers may no longer offer a satisfactory service life and must be replaced with more costly materials which can withstand such an environment.
The work required to compress steam varies with its absolute temperature (Celsius or Rankine). If the final stage temperature is permitted to increase to 750 degrees F. (1210 degrees R.), the work required to compress the steam in that stage increases by over 30 percent compared to the work required to compress the steam at a temperature of about 430 degrees F. (890 degrees R.).
In most compressors, the desired result is an increase in pressure without an excessive temperature increase. In many applications, an excessive outlet temperature is undesirable. Specifications for a turbocompressor which requires an outlet pressure of about 75 PSIA normally limit the superheat of the outlet steam to from about 20 to about 100 degrees F. Normally, with an inlet steam temperature of, for example, about 177 degrees F., the compression process without interstage cooling would raise the temperature to about 750 degrees. This represents an unacceptable superheat of about 440 degrees F. Besides the fact that the superheat is unacceptably high, the other unwanted effects of excessive temperature discussed above are invoked.
In order to reduce the steam temperature in a multi-stage compressor, it is common to employ interstage cooling of various sorts. One type of interstage cooling that has been successfully used is heat exchange cooling wherein the heat is discharged to a cooling medium using a heat exchanger. Heat exchangers are relatively expensive devices which provide relatively poor control of the temperature entering a succeeeding stage.
Another cooling technique which has been successfully used in the past has been the injection of water into the steam between stages. The injected water decreases the steam temperature both by its cooler temperature and by absorption of heat of vaporization as it changes from water to steam. Water injection cooling is relatively inexpensive but it has some drawbacks. The flow path distance from the outlet of one stage of a multi-stage turbocompressor to the inlet of the next stage is relatively short. This short distance makes it difficult to obtain complete conversion of the injected water to steam. If the water is not completely vaporized, however, the remaining solid droplets impinging on the impeller blades of the succeeding stage may, at the least, cause pitting of the impeller blades and, in the extreme, may cause catastrophic failure of the impeller blades.